Contents preface (VII) introduction 1—37

Fig. 2.16 Reservoir capacity from mass-flow curve
Slope of the cumulative demand curve (usually a line since the demand rate is generally constant) is the demand rate which is known. The reservoir is assumed to be full at the beginning of a dry period (i.e., when the withdrawal or demand rate exceeds the rate of inflow into the reservoir) such as A in Fig. 2.16. Draw line AD (i.e., demand line) such that it is tangential to the mass curve at A and has a slope of the demand rate. Obviously, between A and B (where there is maximum difference between the demand line and the mass curve) the demand is larger than the inflow (supply) rate and the reservoir storage would deplete. Between B and D, however, the supply rate is higher than the demand rate and the reservoir would get refilled.
The maximum difference in the ordinates of the demand line and mass curve between A and D

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(i.e., BC) represents the volume of water required as storage in the reservoir to meet the demand from the time the reservoir was full i.e., A in Fig. 2.16. If the mass curve is for a large time period, there may be more than one such duration of dry periods. One can, similarly, obtain the storages required for those durations (EH and IL in Fig. 2.16). The largest of these storages (BC, FG and JK in Fig. 2.16) is the required storage capacity of the reservoir to be provided on the stream in order to meet the demand.

For determining the safe yield of (or maintainable demand by) a reservoir of given capacity one needs to draw tangents from the apex points (A, E and I of Fig. 2.16) such that the maximum difference between the tangent and the mass curve equals the given capacity of the reservoir. The slopes of these tangents equals to the safe yield for the relevant dry period. The smallest slope of these slopes is, obviously, the firm dependable yield of the reservoir.
It should be noted that a reservoir gets refilled only if the demand line intersects the mass curve. Non-intersection of the demand line with the mass curve indicates inflow which is insufficient to meet the given demand. Also, the vertical difference between points D and E represents the spilled volume of water over the spillway.
The losses from reservoir (such as due to evaporation and seepage into the ground or leakage) in a known duration can either be included in the demand rates or deducted from inflow rates.
In practice, demand rates for irrigation, power generation or water supply vary with time. For such situations, mass curve of demand is superposed over the flow-mass curve with proper matching of time. If the reservoir is full at the first intersection of the two curves, the maximum intercept between the two curves represents the required storage capacity of the reservoir to meet the variable demand.
Example 2.4 The following Table gives the mean monthly flows of a stream during a leap year. Determine the minimum storage required to satisfy a demand (inclusive of losses) rate of 50 m³/s.